Feeling worried

I am going to be 37 weeks and 1 day come Monday. I was supposed to have my scheduled c section on Dec. 2nd. But now it is November 25th. Twin A is only 4 lbs 14 oz when they twins got measured on Monday the 18th. Twin B was 5 lbs 1 oz.. I am scared that I am doing something wrong to cause twin A to not be up in her weight. I have a fetal doppler tomorrow I am guessing to check her umbilical cord stuff I am not 100% sure about that. But its scaring me, I dont want either of my girls in the nicu and I deal with the same stuff again I had dealt with 8 years ago. Has anyone have this happen?